Chemical vapor deposited (CVD) and plasma enhanced chemical vapor deposited (PECVD) silicon nitride films have important applications in advanced integrated circuits manufacture. Specific applications include masking layers for local oxidation of silicon (LOCOS), passivation layers and diffusion barriers, and final mechanical protection layers for IC's. A favored method for isolating active regions in advanced CMOS manufacturing is the poly-buffered LOCOS process (PBL), described in Lin, T., N. Tsai and C. Yoo, "Twin-White-Ribbon Effect and Pit Formation Mechanism in PBLOCOS", J. Electrochem. Soc., 138(7), 1991, p 2145, which involves a "stack" of silicon oxide/polysilicon/silicon nitride on a silicon substrate. The silicon oxide "pad" layer is typically 10 nm, while the polysilicon and top silicon nitride layer are typically 50 nm and 250 nm, respectively. After deposition of this composite film over the silicon substrate, active device regions are masked and an anisotropic plasma etching process is typically used to etch the open or "field" regions down to the pad oxide. The wafer is subsequently subjected to a thermal oxidation process for growth of the "field oxide" in the etched areas, which is typically 500 nm in thickness. This process also leaves a thin oxynitride film on top of the silicon nitride. After the field oxide is grown, the stack is removed.
Currently, removal of the PBL stack from the active device regions is done in wet chemical process steps which include an HF acid solution for etching or "deglaze" of the top oxynitride layer, followed by removal of the silicon nitride layer in a hot phosphoric acid solution. The polysilicon layer is subsequently removed in an additional dry etching process, while the "pad" oxide may be stripped using a wet or dry process. There is great impetus for replacing this complicated wet/dry process sequence with a dry method capable of removing the entire PBL stack. This replacement would have benefits not only from the viewpoint of process clusterability, but also from the elimination of hot phosphoric acid from the process sequence. Hot phosphoric acid poses a safety and environmental hazard, is difficult to handle, and is typically one of the most contaminated chemicals in the fabricating laboratory.
In general, a dry LOCOS isolation stack removal process would be required to rapidly etch silicon nitride (and polysilicon), while achieving a selectivity, for silicon nitride over silicon oxide, of greater than 15. The latter requirement limits the removal of field oxide during the nitride stripping process, and prevents thinning or punch through of the pad oxide during an over etch condition.
Dry processes which have been evaluated for nitride LOCOS mask stripping applications include plasma etching, plasma downstream etching, and plasmaless etching of silicon nitride using fluorine interhalogen and other spontaneously reactive gases.
Akiya, Proc. of Dry. Proc. Symp., Oct. 1981, Tokyo, p 19, demonstrated in a plasma beam experiment that F atoms produced in an upstream CF.sub.4 --O.sub.2 RF discharge spontaneously etch silicon nitride (Si.sub.3 N.sub.4) at appreciable rates, while thermal SiO.sub.2 and PSG (phosphorous doped silica glass) were etched much more slowly.
Sanders et al., J. Electrochem. Soc., 129(11), 1982, p 2559, studied the selective isotropic dry etching of Si.sub.3 N.sub.4 over SiO.sub.2 using CF.sub.4 --O.sub.2 mixtures in a commercial barrel etcher, and found that additions of CF.sub.3 Br increased selectivity for the nitride from 5 to over 20. They further suggested that there was an increasing effect on nitride selectivity as one added other halogens to the base fluorine chemistry, in the order of chlorine, bromine, iodine.
Suto et al. J. Electrochem. Soc. 136(7), 1989, p 2032, studied Si.sub.3 N.sub.4 to SiO.sub.2 selectivity in a downstream microwave plasma process, where additions of Cl.sub.2 to a NF.sub.3 discharge chemistry were found to greatly enhance nitride selectivity.
Lowenstein, et al., J. Vac. Sci. Technol. A, 7(3), 1989, p 686; J. Electrochem. Soc., 138(5), 1991, p 1389; and Proc. of ECS, 93(21), 1993, p 373, have characterized the etching of LOCOS isolation structures in a microwave-based remote plasma reactor. In these references, the effect of substrate temperature, as well as hydrogen additions on the etching selectivity of silicon nitride to thermal SiO.sub.2 and polysilicon, have been detailed and the removal of silicon oxynitride in a similar dry process was also compared to conventional wet hot phosphoric acid process. All of these references, however, pertain to plasma etch systems. Direct plasma etch systems are generally unacceptable due to various types of charge and ballistic damages which are created in the active device regions. Downstream plasma systems for dry stripping of silicon nitride which expose the substrate to plasma effluent, rather than to the plasma glow region, have disadvantages which include the deposition of sputtered contaminants in the plasma tube onto the wafer, the strong effect of transport tube material and geometry on the wafer chemistry, the exposure of the wafer to residual charge flux and long lived high energy metastables, and process sensitivity to reactor conditioning effects. Accordingly there is a need for a dry plasma-free process for stripping silicon nitride from wafer substrates.
Ibbotson et al., Appl. Phys. Lett., 46(10), 1984 p 2939, demonstrated that plasma deposited silicon nitride and LPCVD Si.sub.3 N.sub.4 could be etched at an appreciable rate in a plasma-free process using only vapors of chlorine trifluoride (ClF.sub.3), while thermal SiO.sub.2 was not etched at detectable rates.
Saito et al., IEICE Trans. Electron, E75-C(7), July 1992, p834, have further studied the "plasmaless etching" of thermally grown, sputtered, and plasma deposited silicon nitride films with ClF.sub.3 vapors. Low intensity UV exposure of thermal silicon nitride during ClF.sub.3 etching was found to reduce the induction time before the onset of etching, and to increase ClF.sub.3 --Si.sub.3 N.sub.4 etching rates by about a factor of 2. Under the conditions studied, selectivity of Si.sub.3 N.sub.4 was reported to be greater than 100, however, the nitride etching rates reported were under 5 .ANG./min.
In copending application Ser. No. 08/259,542, filed Jun. 14, 1994, it is disclosed that selectivity between various forms of silicon oxide is reduced to a factor of near 1:1 when a substrate containing several types of silicon oxide is exposed to a plasma-free gaseous environment comprising a photodisassociable fluorine containing gas and irradiated with UV.